__________________"The world is like a ride at an amusement park. And when you choose to go on it, you think it's real because that's how powerful our minds are. And the ride goes up and down and round and round. It has thrills and chills and it's very brightly colored and it's very loud and it's fun, for a while. Some people have been on the ride for a long time, and they begin to question: Is this real, or is this just a ride? And other people have remembered, and they come back to us, they say, "Hey, don't worry, don't be afraid ever, because this is just a ride." And we ... kill those people." - William Melvin Hicks

CO2 tanks: 2 inches or less in diameter and less than 2 feet long, do not need to be tested. This is a DOT Rule. These CO2 bottles usually have a 1800 psi limit. Greater than 2 inches in diameter must be tested every 5 years. These bottles are not fiber wrapped. These are considered high pressure.
FYI: Exemption Numbers That number on your tank that starts with "E-" does not mean the bottle is exempt from testing, it means the manufacturer is required to have a current authorization letter on file with the DOT. These letters describe what requirements the DOT and the manufacturers have placed on the bottle's use, the testing requirements and other important information. There are some stores and paintball web sites that say these "E" bottles are from testing. NO they are not exempt from testing, all "E" bottles must be tested (requalified).

About the "less than 2 inches in diameter and under 2 feet in length" re-hydro exception:
Not a single person on any paintball forum (or anywhere on the web, thus far) was able to provide a definitive link to a Code of Federal Regulations, or even cite a Department of Transportation representative, stating this exception to be true. I took it upon myself to fully investigate this. I have called today the Department of Transportation in Washington DC [(202) 366-2301], and asked to speak to the HazMat Division (that's who handles the gas cylinders). The representative has pointed me to Title 49, section 178 and Title 49, section 180.209 of the CFR (Code of Federal Regulations), where id definitively states that ALL CYLINDERS MUST BE HYDROTESTED (re-certified) unless the cylinder was made under specification "4L" (see the table in Title 49, section 180.209 of the CFR).
Personally, I have never seen a tank under 2 inches in diameter with a "4L" stamp on it. They're all "DOT"-something, that I've seen: either the 9 oz CO2, or the 10 oz CO2 tanks, or the 13 cubic inch HPA tanks, or the 22 ci HPA tanks (which all are under 2 inches in diameter and less than 2 feet long).
Thus, the bottom line is this: ALL TANKS NEED TO BE HYDRO-TESTED. THE ONLY EXCEPTION ARE TANKS MARKED "4L". The "under 2 inch diameter bla-bla-bla" exception is FALSE.
All being said, however, it is nice that the field operators are still human beings and keep that "under 2 inch in diameter" exception myth as a rule and fill those small tanks regardless of hydro expiration date.

Can anyone confirm this with a link/primary source? Would this mean HPA tanks stamped DOT-3AL still need hydro testing? I would like to know this once and for all!

Yes, it technically would need to have re-certification done every 5 years (see CFR links above). However, most fields and shops will ignore that, and just follow that "under 2 inch ..." rumor rule. Which is a good thing, imho. Just mind
1) damaging the tank and blowing up yourself and/or others. and
2) if you're traveling, or if someone does check your equipment for safety compliance - be aware that you will not pass.

Yes, it technically would need to have re-certification done every 5 years (see CFR links above). However, most fields and shops will ignore that, and just follow that "under 2 inch ..." rumor rule. Which is a good thing, imho. Just mind
1) damaging the tank and blowing up yourself and/or others. and
2) if you're traveling, or if someone does check your equipment for safety compliance - be aware that you will not pass.

Call HazMat division up and ask them. Seriously. Name this document and see what they say.

Well, either way, you can't really blame the field operators when it is written in the same table you were told to look at. Exception 1 is applied to the whole table, not just 3AL. It has also been there for several years, so I doubt it is misplaced.

Actually, that table in Title 49, section 180.209 of the Code of Federal Regulations has the same notation "1" that reads Any cylinder not exceeding 2 inches outside diameter and less than 2 feet in length is excepted from volumetric expansion test.. I'll call up HazMat myself and confirm that.